My first pet was a comet goldfish I christened Ralph, won at my local carnival this past July. Although my mother wasn't enthused about the sudden existence of a pet (we've been arguing about getting a cat for years now--I wanted one really badly) she didn't protest when I came home with his bag, some food, and a little temporary dollar-store bowl in hand. I let him out into his new home, and he was eager to explore. This was a Saturday--the ninth, if I remember correctly.
The next morning, after a fun night of research, I went to PetCo and bought him a one gallon bowl with marbles and an artificial plant for him to live in until he began growing, for I'd found out that goldfish grow to at the very least six inches. I filled up the bowl, treated the water and let it reach room temperature before letting him in there. (this was before I'd learned about acclimating and such) He swam around excitedly the first few hours, and when I went to check on him later that night he was sitting at the bottom of the bowl. Since I hadn't fed him that morning, I dropped in a few flakes. He waited until they floated down before nibbling at them, but promptly spit them back out to drift down between the marbles. As this continued, I grew concerned about him. This was Sunday.
I changed the water the next morning before leaving for my swim class and fed him. He kept the flakes inside this time, but I found the mushy remains of the food on the marbles below when I got home. I sprinkled in some more flakes, willing him to eat. After watching him idle at the bottom of the bowl behind the plant, I left, having failed, and made lunch. At two o' clock, I transferred him to his little old bowl and toted him gently and slowly around the house with me as I did my chores. I noticed a few scales on his side near his dorsal fin had turned black. When finished, I retreated to my room and rested his bowl on my bedside table where I glanced at him every ten seconds or so while I played on my DS.
He stopped swimming then.
I dipped my finger in the water and gently poked him, which made him feebly dart away, but drift on his side. I repeated the process maybe six more times, growing more and more distressed each time he moved less, and eventually, not at all. I can honestly say I shouted at him to keep moving. I panicked, and ran carefully back to his bowl and dropped him in, this time nudging him with the net. I was in emotional shock, and kept denying that he was gone, but it was the truth and I had to live with it.
To this day, it still bothers me--I never knew how or why he died. Could it have been that my several attempts to feed him unsuccessfully led to a lethal buildup of ammonia in his water, thus causing his ammonia burns (assuming that's what the blackened scales were) and probable poisoning? I was still not well-versed in the field of fish-keeping, as it was only my second full day of doing so.

tl;dr: He idled at the bottom of the tank after being pretty active for a while, and stayed down there, refusing to eat. A couple of his scales turned black on the second day, and he died the third. Ralph was an inch long comet goldfish, housed in an unfiltered one gallon bowl.

He was a carnival fish, so maybe he had some pre-existing disease...Or maybe I'm just being hopeful.
Judging by the symptoms described, do you think I made a lethal mistake or was it a stroke of bad luck? Don't spare my feelings.
Also, room temperature this summer was about 73 degrees Fahrenheit.

Just a common mistake :) don't worry... What it is from, is ammonia burn/poison, and oxygen depletion. Many people do not know it, but goldfish are very very messy!! They poop a lot, and poop = waste = ammonia. Ammonia, is lethal to all fish - goldfish especially. Their poo, kills them. That's right. Death by poo (eew). So, to make the waste less, you have more water. say... for the bowl it was.... 5 parts waste, 1 parts water. That's a lot right? (excuse my lame math skills) well, then you add.... 20 parts water. now ammonia is very very weak, and not as lethal.

Cycling is also important, on the topic of ammonia. Cycle involves ammonia, nitrite and nitrate. these, are bad for fish but... ammonia breaks down during cycling (takes 4-6 weeks) which makes it non-lethal. So, for goldfish, despite their itty bittiness, have a huge "bioload" (waste) and they do get really big - plus they should live between 10-25 years! depending on the quality, care, size of tank, etc. Temperature doesn't matter too much - like a betta who needs 78 degrees!! Goldfish love colder water, show their best colors in colder water.... many people keep them at room temperature (around 75).

So, goldfish needs: anywhere between 30-50 gallons, depending on type. this is for ONE goldfish. two, need about 70, three 100+. Why? because of bioload, and oxygen levels. Unlike betta fish who have the labrynth organ, goldfish need oxygen. Many people have heavier air flows for their tanks for goldfish, or even double filtration.

I don't blame you... it's a highly common mistake. I'm not going to come at you "blah you are a bad person sjkhgrebggw" because it is too common, and misinformation leads people to do exactly what you have done - a simple mistake.

I second Sena, in a small bowl, the ammonia from the goldie built up extremely fast. It's an honest mistake. They look really small but they are very productive with the waste. You're learning as you go. Now you'll know all the info you need to own a goldfish in the future, thanks to Sena. :)

It happens :) Like I said, very very common. I wanted a black moore goldfish (still do!!! <3 ehehe buggy eyes..) But I won't because I know they are more costly tank wise, and more energy is needed than a betta x.x and bettas are hard enough hehe